Well, it doesn't sound like you've got too many options unless your days are going to move later and later all the time! I would keep waking from the first nap (just my preference - so that if the first nap goes bad or is skipped, there is a pm nap to catch up with), and I expect he will get used to it. You could go into his room at 25 mins and wake him when he is stirring, to see if that helps?